In the prior filed but later published patent application with the official application number 10 2005 017 489.2, the dose data value assigned to the p-quantile of the image is used as the actual value for the dose or dose rate in an X-ray image.
A p-quantile is a value obtained from a frequency distribution, assuming a frequency distribution in which a plurality of values of a particular quantity are assigned a particular frequency. If the frequency distribution has n entries in total, the p-quantile (with 0<p<1) is the value at which n*p entries in the frequency distribution are entries for a lesser (or equal) value, and (1−p)*n entries pertain to values which are greater than the p-quantile.
When looking for the p-quantile, one is therefore looking for a value which divides the frequency distribution in a particular ratio.
In x-ray systems according to the prior art, as a rule the image is not now analyzed as an entity.
Instead, the image is divided into disjoint areas, typically into four disjoint image areas. Each of these image areas is assigned a (digital) signal processor. The signal processors allow parallel processing of pixel data from the respective image areas, so that a central processor only receives data from the signal processors on the basis of which it can “complete” image analysis. In other words, either preparation for image analysis or even part of image analysis takes place in the signal processors, and the combining of the data obtained by the signal processors is then the responsibility of a central processor (LCPU, Local Central Processing Unit).
However, this division of responsibilities among different signal processors in modern x-ray machines is a priori obstructive to determining a p-quantile. As the p-quantile is not a computationally determined value but a statistically determined value, it requires information concerning the frequency distribution.
As a first step, the individual signal processors would each normally be used to generate a frequency table in which, for the digital image, every pixel value is assigned the frequency of its occurrence in the image area associated with the signal processor.
These frequency tables would then be combined to form a total frequency table. As the pixel levels are predefined as such, only the assigned table entry needs to be added in each case.
The reason for associated problems is that modern x-ray imaging systems provide a large number of possible gray levels (which correspond to dose levels). 16 bits are typically used for the gray levels. Also in other imaging systems the number of pixel levels (these can also be color levels) is very high.
16 bits mean that there are 216 grayscale levels, i.e. approximately 64000 grayscale levels.
However, this also means that the frequency table has 64000 entries.
If the frequency table is to be transferred from the signal processor to the main processor, four times 64000 value assignments must be transferred. However, each individual data transfer requires approx. three clock pulses. If data analysis with determination of the p-quantile is to take place in real-time, this is critical in x-ray systems in which e.g. 30 images are generated per second. Because of the large number of data entries to be transferred for the frequency table, real-time analysis is no longer guaranteed.